In oil production, refining, and treating, it is frequently necessary to meter precisely controlled amounts of additives to feed, process, or output streams. Since these streams are often at very high pressures, it is necessary to meter the input at the same, or greater, high pressures. It becomes very difficult to meter precisely controlled volumes of liquids at high pressures because of the necessity to bring the metered liquid to a precisely controlled pressure at least equal to the pressure of the stream into which it is metered and to maintain that pressure. Serious perturbations and consequent errors typically result in processes using prior art metering pumps.
Prior art metering pumps typically have been extremely heavy devices, very expensive, difficult to handle, and cumbersome in operation. These pumps may, for example, weight many hundreds of pounds and may be extremely difficult to install and to maintain. One reason for the large size of these pumps is that, in an effort to minimize the number of serious pressure perturbations which occurred, large pistons were used so that once the system was at pressure, the additive could be metered out of the piston for a comparatively long period of time before it was necessary to refill the piston or transfer to another metering pump. Large pistons, of course, require very high forces to obtain the desired high pressures. This means that heavy duty support systems, brackets, bearings, drives, etc., are required. Consequently, every component must be heavy and interconnected very solidly. This results in very expensive and cumbersome units.
There have been some efforts to use smaller metering pumps, and some efforts have been made to minimize perturbations in the system. For example, the use of double screw type pumps have been proposed. This involves two pumps for compressing a liquid at a constant speed, by rotating a screw for moving a liquid transferring piston by a pulse motor or servo motor through gears, so that a supply and a suction of the liquid are alternately provided to prevent intervals therebetween. This system, however, requires various techniques and labor for preventing inaccurate timing in the switching process. Thus, even though accurate conformity of the characteristics of the two pumps is provided and accurate simultaneous switching is possible, a pressure fluctuation still results, corresponding to a different coefficient of a change in the room temperature when a solvent having a high thermal expansion coefficient is used. In an effort to overcome these problems, Sakiyama, et al., U.S. Pat. No. 3,847,507, Nov. 12, 1974, provided a system for supporting a liquid by pump when the pressure in a cylinder of the pump is detected and the movement of the piston is controlled by an automatic control circuit having a differential amplifier through an electric motor and feedback signal originated from a tachometer connected to the motor.
The use of screw driven pumps is, of course, well known, and even the low friction ball-screw mechanism has been used in driving pumps. See, for example, U.S. Pat. No. 3,397,643, Jepsen, Aug. 20, 1968, and Glasgow, U.S. Pat. No. 3,208,388, Sept. 28, 1965. Other screw driven metering pumps, some including control mechanisms, are disclosed in U.S. Pat. No. 4,276,003, Perkins, et al., June 30, 1981, U.S. Pat. No. 3,255,096, Coker, Jr., et al., Mar. 10, 1981, and U.S. Pat. No. 3,556,679, Middlebusher, et al., Jan. 19, 1971.
A number of control mechanisms and stepping motor driven pumping systems have also been used. See, for example, U.S. Pat. No. 4,326,837, Gilson, et al., Apr. 27, 1982, U.S. Pat. No. 3,653,787, Commarmot, Apr. 4, 1972, U.S. Pat. No. 4,304,527, Jewell, et al., Dec. 8, 1981, U.S. Pat. No. 3,775,025, Maher, Jr., et al., Nov. 27, 1973, and U.S. Pat. No. 3,814,541, Dent, et al., June 4, 1974.
In the prior art pumps generally, the pumping is accomplished by a cylinder in which a piston reciprocates, thus giving a precise displacement for each stroke, assuming constant pressure. The piston is driven, typically, from an electric motor connected to a screw or cam drive through a complex gearing or chain drive mechanism resulting in a cumbersome, heavy, and often inefficient pumping system.
The present invention has as one of its objects and features, providing a light weight, highly efficient, extremely accurate metering pump in which the drive motor is integral with the piston and drive screw.